Unauthorized copying of sensitive information has occurred since man has been storing and tracking information on documents. Combating document fraud and especially protecting printed content is a multidisciplinary and international concern. The constant improvement of modern scanners with digital means of signal processing and color copiers has made it economically feasible to reproduce almost perfect looking reproductions of currency and fraudulent travel documents. This same technology is now used every day to easily copy printed information for nefarious reasons.
Many different security printing techniques have been developed to alert a casual observer of a copy attempt at the time of general inspection. These technologies are referred to in general terms as “copy evident” because the photocopy is produced with a warning message. While these “copy evident” techniques might provide some level of warning on a copy, they do not however stop a criminal from identifying, copying and sharing valuable private information.
Years ago, copiers used to be manufactured in a way as to black-out the copies if they encountered a red ‘CONFIDENTIAL’ on the document. The black-out was achieved by tuning a lamp so as to cause a reflection of the color red. This technology quickly became obsolete as some tried to detune to lamp.
In accordance with various copy protection techniques, a hidden warning message, such as ‘VOID’ or ‘COPY’, is printed in a halftone within a halftone background on a substrate. The line screen value of the hidden warning message is selected so that the elements of the hidden warning message are reproduced when photocopied. The line screen value of the background, however, is selected, such that the elements of the background are much smaller and would not easily reproduce when photocopied. As a result, and in theory, the hidden warning message will appear on duplicates of the original document made by photocopying. This method is also sometimes used by reversing the screen values of the hidden warning message and the background, such that the elements of the hidden warning message are not reproduced, and the elements of the background are reproduced when photocopied or scanned.
While the above techniques have provided some degree of copy-evident protection of original documents with respect to most copiers, in recent years digital scanners and color copiers continue to improve both their resolution and digital filtering capabilities substantially. These new color copiers and scanners can reproduce at a very high resolution of 600×1200 or higher and have made the above techniques less effective in protecting original documents.
In most cases the hidden warning message does not readily appear on the reproduction of the original document, so that a casual observer of the document may not be alerted that the document in possession is not the original. A greater element disparity between the respective line screen values and tonal screen values of the hidden warning message and background pattern would allow the hidden warning message to appear on a reproduction of the original document even with the manipulation of the copier. Due to this disparity, however, most presently known camouflage techniques may not adequately suppress the visual appearance of the hidden warning message on the original document being rejected as a copy, which would not be acceptable to issuers of the original. Despite the various copy protection technologies known, what is needed is a superior strategy for controlling the distribution of documents and portions thereof.
Therefore, it would be desirable to provide a strategy for controlling copying of documents that would overcome the aforementioned and other disadvantages.